1. Field of the Invention
The present invention relates to a method for nondestructively measuring and quantitatively evaluating deterioration of material strength due to aging, in ferromagnetic construction materials, or in structures comprised of such materials.
2. Description of the Related Art
Conventional nondestructive inspection methods for aged material deterioration had generally aimed at investigating of initiation and growth of cracks in the material in almost every case. And thus, the direction of development in present nondestructive inspection methods lies in finding out produced cracks as minute as possible. Accordingly, with such a conventional nondestructive inspection method, it is practically impossible to inspect nondestructively aged deterioration of materials before the initiation of cracks.
By the way, another type of previous method is known for nondestructively measuring material strength deteriorated with age in ferromagnetic construction materials or structures comprised in such construction materials. In this measuring method, the coercive force and magnetic susceptibility in the range approaching to saturation magnetization of a measuring object are measured.
However, in the above-mentioned conventional method, a magnetic field that is far larger than the coercive force of materials has to be applied for the magnetizing until the magnetization is saturated. Consequently, it is necessary for using a large magnetizing yoke and allowing a large magnetizing current to flow through the magnetizing coil. Therefore a measuring machine incorporating such a large magnetic yoke and a large capacity magnetizing power source for energizing the magnetic yoke is not only expensive, but also makes the entire system heavy and large in size to require a noticeable installation space.
Consequently, in order to solve these problems, the Inventor proposed a method for evaluating aged deterioration in evaluating ferromagnetic construction materials, or in structures comprised of such ferromagnetic construction materials, which is based on relationships between the stress σ and the susceptibility coefficient c (=χcH3) obtained from the magnetic susceptibility χc and the magnetic field intensity H of an evaluating material (see, for example, Japanese Examined Patent Publication No. 158182), and another method for the same purpose which is based on relationships between the ratio of the coercive force of an evaluating material against its magnetic susceptibility, and the coercive force (see, for example, Japanese Examined Patent Publication No. 3300810).
However, it has been impossible to obtain detailed information regarding the aged deterioration of ferromagnetic construction materials by using such methods as mentioned above which are dependent on the measurement of the coercive force and magnetic susceptibility of the material. The reason is as follows. Both the coercive force and the ratio of the coercive force against the magnetic susceptibility obtained from hysteresis loops, give the information which appears according to the maximum force which the magnetic domain walls receive from a lattice defect in the movements of the magnetic domain walls, in the state in which the maximum of the magnetic field intensity applied to that material reaches saturation magnetic field intensity, and the domain walls inside that material becomes the most ready to move out.
The susceptibility coefficient c also is the information in a limited range of magnetization of the evaluating ferromagnetic material. Accordingly, these parameters provide limited information about the aged deterioration of an evaluating ferromagnetic material. Therefore, according to the above-mentioned conventional methods, although information regarding the aged deterioration of a material by using the coercive force is obtained from a maximum potential energy for domain wall movements in the material, it is impossible to account for an overall image of the potential energy. Any one of the above-mentioned methods can hardly specify, with regard to the aged deterioration of the evaluating ferromagnetic material, individual defects in terms of the internal structure of the evaluating material responsible for the deterioration.
Moreover, in the case of evaluating the aged deterioration of an evaluating ferromagnetic material using the ratio of its coercive force against its magnetic susceptibility, it is necessary to apply, to the evaluating material, the comparatively high magnetic field intensity more than its coercive force of the evaluating material. Further, to exactly determine the aged deterioration of an evaluating ferromagnetic material by the method dependent on the susceptibility coefficient c, in order to evaluate the aged deterioration of ferromagnetic materials more correctly, obtaining an ideally shaped hysteresis loop of the material is desirable. For this reason, it is difficult to make the maximum level of the magnetic field intensity still lower. Thus, with both methods, if the externally applied magnetic field is lower than specified above, it is hardly possible to obtain precise and detailed information about the aged deterioration of an evaluating ferromagnetic material.